In a conventional differential amplifier circuit, current from a constant current source is divided between two current paths on the basis of two input voltages. Any current difference resulting thereby is reflected in voltages produced across equal loads located in the two current paths. For a differential amplifier circuit fabricated in a semiconductor integrated circuit, the constant current source is usually realized by one or more transistors and each current path consists of a differential input transistor and a load transistor connected in series. The load transistors are coupled together to form a current mirror load, thus yielding relatively high gain. High input impedence output amplifier stages are typically added to the differential amplifier in a practical circuit application, such as a voltage comparator.
A problem arises in the conventional differential amplifier circuit having a constant current source for applications where a low supply voltage is encountered. Specifically, the minimum supply voltage is limited to approximately the sum of the series-connected transistor threshold voltages, and the common mode input range is limited by the requirement that the transistor implementing the constant current remain in saturation. In automotive applications, for example, the upper limit of the common mode range for a conventional circuit is typically only slightly more than one half the supply voltage. Furthermore, circuits designed to operate from a low supply voltage exhibit poor speed performance due to increased capacitance accompanying the necessary increase in the size of the circuit devices. A further disadvantage of the requirement of a constant current source is increased overall power consumption and limitations on the current available to various circuit elements during circuit operation. It is desirable, therefore, to provide a low voltage, low power, high speed, precision differential amplifier circuit for which the common mode input range approaches the supply voltage.